In the film, Larson plays Ma, mother to five-year-old Jack, who has lived every day of his life in what he calls “Room.” To Jack, “Room” is the entire universe. Ma has kept from him that there is a world outside, a world she used to live in before she was kidnapped by a man known as Old Nick.

Larson conveys the profound love Ma has for Jack while never losing sight of her frustration, anger, and pain. Ma has turned Room into an almost idyllic haven for her child, while deep down knowing that he is experiencing only a tragic sliver of what life has to offer.

I was hoping to love this film and had high expectations; I'm a big fan of Emma Donoghue's book on which it is based. Donoghue, who also wrote the screenplay, raved to me about Larson’s performance. Last month, I talked to Donoghue about the movie, and she explained how Larson wrote Ma's teenage diaries and on screen, “creates an archetype of female power."

During a Q&A following the screening, Larson mimed patting her head and rubbing her stomach to describe what she thought building the character would be like. She then explained that she and director Lenny Abrahamson met with a trauma specialist. “He was able to discuss with us how the brain works and how the brain shuts off certain awarenesses when things are too much for the body to handle because the body wants to continue to live,” she explained. “So it does these things like shuts down certain fearful things in order to protect itself.”

They also spoke with a doctor about how a lack of vitamin D would affect a person, and they considered how Ma’s life would be affected by poor nutrition, living without a toothbrush for a time, sexual abuse, and a pregnancy. “These were all just like tons of little puzzle pieces,” Larson said. “They all were individual until I was able to put them all together. That became the structure of her."

But Ma wouldn’t be complete without Jack, and eight-year-old Jacob Tremblay is fantastic. After Larson developed her understanding of Ma, she and Tremblay got to the business of establishing the connection between mother and son.

“We just played in Room, we would just go into Room every day for, like, two weeks,” Larson said. “Just hang out. Sometimes Lenny would be in there; sometimes he would close the door and just let us be in there. We were able to create a really fun space. We had a really awesome time together. He’s my really good friend and I enjoy spending time with him.”

That much was clear. Just before Tremblay joined the rest of the cast for the Q&A, Larson was looking offstage and smiling, and she kept engaging with him throughout the rest of the event.

During an interview over the summer, Donoghue said: "It is crucial that Brie established a real rapport with him in a way you just can’t fake. So he and she were laughing and teasing each other a lot. This gave them a kind of intimacy that you couldn't just turn on as a performance. His performance is astonishing too and quite a lot of it is thanks to Brie." It was such a delight to see that used to dramatic effect in the film, and to charming effect on stage.